Method, device and system for the exchange of data via a bus system

ABSTRACT

A method is described for exchanging data in messages between at least two stations connected via a bus system, the messages containing the data being transmitted by the stations over the bus system and the messages being controlled over time by a first station in such a manner that the first station repeatedly transmits a reference message containing time information of the first station over the bus system at at least one specifiable time interval, the time interval being subdivided as a basic cycle into time windows of specifiable length and the messages being transmitted in the time windows, in which method, when data is exchanged, a pause period of variable duration is provided at the end of at least one basic cycle, by which a time change of the beginning of the basic cycle is corrected by adaptation of the duration of the pause period.

FIELD OF THE INVENTION

The present invention relates to a method, device, and a system forexchanging data such as in messages between at least two stationsconnected via a bus system.

BACKGROUND INFORMATION

The networking of control units, sensors and actuators using acommunications system or a bus system has increased dramatically inrecent years not only in modern motor vehicle manufacturing and inengineering, especially in the machine tool sector, and automationtechnology and other industrial applications, but also in the privatesector, for example in bus systems for domestic buildings. It ispossible in these cases to obtain synergetic effects by distributingfunctions among several control units. The term distributed systems isused for this. Communication between various stations of such a systemis increasingly taking place via at least one bus or at least one bussystem. The communications traffic on the bus system, access andreceiving mechanisms, and error handling are governed by a protocol.

A protocol that is established in the automotive sector and which isalso being used to an increasingly greater extent in other applicationsis CAN (Controller Area Network). This is an event-triggered protocol,that is to say, protocol activities such as transmission of a messageare initiated by events that originate outside the communicationssystem. Unique access to the communications system or bus system isresolved by priority-based bit arbitration. A pre-requisite for this isthat each message be assigned a priority. The CAN protocol is veryflexible; it is therefore possible for further nodes and messages to beadded without any difficulty as long as there are still free priorities(message identifiers) available. The collection of all of the messagesto be transmitted in the network, including priorities and theirtransmitting nodes, and possibly receiving nodes, are stored in a listknown as the communication matrix.

An alternative approach to event-triggered, spontaneous communication isthe purely time-triggered approach. All communication activities on thebus are in that case strictly periodic. Protocol activities such as thetransmission of a message are triggered only by the passage of a timeapplicable to the entire bus system. Access to the medium is based onthe allocation of time ranges in which a transmitting station has anexclusive transmission right. The protocol is comparatively inflexible,and adding new nodes is possible only if the corresponding time rangeswere left free beforehand. This circumstance forces the order of themessages to be set before operation is started. At the same time, thepositioning of the messages within the transmission periods must also besynchronized with the applications producing the contents of themessages so that the latencies between the application and the instantof transmission are kept to a minimum; otherwise, that is to say, ifthat synchronization is not performed, the advantage of time-triggeredtransmission—minimal latency jitters when the message is being sent overthe bus—would be destroyed.

The approach using time-triggered CAN, the so-called TTCAN (TimeTriggered Controller Area Network), which is described in German PatentApplication Nos. 100 00 302, 100 00 303, 100 00.304 and 100 00 305, andin ISO Standard 11898-4 satisfies the requirements outlined above fortime-triggered communication and satisfies the requirements for acertain degree of flexibility. The TTCAN fulfills those requirements bystructuring the communication round (basic cycle) into so-calledexclusive time windows for periodic messages of specific communicationsstations, and into so-called arbitrating time windows for spontaneousmessages of a plurality of communications stations. The TTCAN isgenerally based on time-triggered, periodic communication which isclocked by a station or node giving the main time, the so-called timemaster or timer, using a time reference message or short referencemessage. The period to the next reference message is referred to as thebasic cycle and is subdivided into a specifiable number of time windows.A distinction is made between the local times, or the local timers, ofthe individual stations and the time of the timer giving the globaltime. Further fundamental principles and definitions relating to theTTCAN will be explained hereinafter or may be learned from ISO 11898-4and the related art described above.

In the case of TTCAN bus communication, communication objects,especially messages, that are defective and that are marked and madeinvalid by an error frame are not repeated so as to avoid any risk ofexceeding the time window or the cycle time by repeating the message andthereby impeding the message that follows. The receiving communicationobject for that destroyed message continues not to be updated until amessage is received without error in an associated time window. Incontrast to this, a defective reference message identified by an errorframe is repeated, since it is not possible to do without that referencemessage. That repetition of the message results in the basic cycleaffected being extended by the time from the beginning of the firstdefective reference message to the beginning of the reference messagetransmitted without error. Each of those errors leads to a further delayin the timing, with the result that those delays add up to a greater andgreater deviation from the nominal time. Such a fault, for example areference message transmitted with errors, accordingly leads to a timechange or deviation from the nominal settings in the system. If two ormore TTCAN buses or bus systems are in synchronized operation, such atime deviation, especially a delay, on one of the bus systems must beput into effect on the other bus system in order to obtain synchronismagain. Accordingly, such time deviations on all the bus systems areadded to one another and the fault or error is propagated.

SUMMARY

An object of the present invention is to compensate for such timedeviations, especially delays, caused by faults and thereby to obtaingreater long-term accuracy of the timing of the communication and thecommunication matrix.

An example embodiment according to the present invention is notnecessarily confined to the TTCAN, but may be extended to comparable bussystems and protocols as regards the requirements and constraintsdescribed hereinafter. For a clearer understanding, however, the TTCANbus is taken as the basis for the following description.

In accordance with an example embodiment of the present invention, amethod, device, and corresponding bus system, are provided forexchanging data in messages between at least two stations connected viaa bus system. The messages contain the data being transmitted by thestations over the bus system, and the messages are controlled over timeby a first station in such a manner that the first station repeatedlytransmits a reference message containing time information of the firststation over the bus system at at least one specifiable time interval.The time interval is subdivided as a basic cycle into time windows ofspecifiable length. The messages are transmitted in the time windows.When data is exchanged, a pause period of variable duration may beprovided at the end of at least one basic cycle, by which a time changeof the beginning of the basic cycle is corrected by adaptation of theduration of the pause period. It is thereby possible to handle theabove-described problems in the event of deviations with regard to thecycle time in one or more bus systems.

The time change in the form of a delay in the start of the basic cycleis advantageously corrected by shortening the duration of at least onepause period.

In different forms of application, a pause period may be provided at theend of every basic cycle or at the end of every 2^(n)th basic cycle orat the end of every 2^(n+)1th basic cycle, where n is a natural number(n ε N).

When a plurality of, i.e., at least two, successive basic cycles areconsidered, it is also possible to provide a plurality of pause periods,appropriate to the different forms of application, so that a time changeof the beginning of at least one basic cycle may be distributed over aplurality of, and especially at least two, pause periods, and acorrection may thereby be made.

A correction value may advantageously be determined for this, which isfound from a local time of a station and a cycle time. The correctionvalue is advantageously determined from a first difference between twolocal times of a station in two successive basic cycles. In addition,the correction value is dependent on a second difference between twocycle times of two successive basic cycles. The correction value mayalso advantageously be dependent on a comparison value formed by the sumof the time interval of the basic cycle and the above-mentioned seconddifference, so that the correction value corresponds to the differencebetween the first difference and the comparison value.

It is thus advantageously possible, when at least two pause periods areused in at least two successive basic cycles when exchanging data, forthe correction value to be distributed in a specifiable manner over theat least two pause periods, so that, if the duration of a pause periodis not sufficient to correct the time deviation, time compensation isalso possible over a plurality of pause periods and basic cycles. Inparticular, in this instance the correction value may be evenlydistributed over the at least two pause periods rather than beingdistributed in a specifiable manner.

Further advantages and advantageous embodiments will be apparent fromthe description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in detail below with reference to theFigures.

FIG. 1 shows a bus system having a plurality of stations.

FIG. 2 shows a system having two bus systems which are coupled to eachother.

FIG. 3 shows a total cycle for exchanging data, having a plurality ofbasic cycles.

FIGS. 4 a and 4 b show the effect of a fault without the pause periodaccording to an example embodiment of the present invention.

FIGS. 5 a and 5 b show the correction of a fault using the pause periodaccording to an example embodiment of the present invention.

FIGS. 6 a and 6 b show a plurality of successive basic cycles with acorrection according to an example embodiment of the present inventionbeing determined.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

TTCAN is generally based on time-triggered periodic communication whichis clocked by a timer (FIG. 1, 101) using a time reference message orshort reference message RN. The time interval or the period up to thenext reference message RN has the duration of cycle time ZZ. FIG. 1shows a bus system 100 having a plurality of bus stations 101 to 105.Each station 101 to 105 has its own time basis 106 to 110 which on theone hand may be formed by an internal element, for example a clock, acounter, a clock generator etc. or may be transmitted to the respectivestation from the outside. The respective local time basis LZ1 to LZ4 is,for example, especially a counter, for example a 16-bit incrementalcounter, that may be influenced only by a hard reset. A local time basisis implemented here in each station 102 to 105. One specific station,the timer, in this case 101, has an exposed position. Its time basis 106is referred to as the global time basis having the global time GZ and iseither implemented in the timer 101 or transmitted thereto from theoutside. The global time GZ is formed in principle in each station bythe local time basis 107 to 110, or the local time LZ (LZ1 to LZ4), andan offset OS1 to OS4. That offset OSG in the timer 101 is normally 0(OSG=0). All the other stations form their view of the global time GZfrom their local time LZ (LZ1 to LZ4) and the local offset OS1 to OS4(and in exceptional cases from OSG when OSG≠0, for example, when GZ istransmitted to timer 101 from the outside and the latter additionallycontains its own time basis). The local offset is the difference betweenthe local time at the instant of transmission (SOF, start of frame), ofreference message RN and the global time information transmitted fromthe timer in or with that reference message RN. To determine specificpoints in time, for example to determine the offset relative to therespective local time and the global time, so-called time marks(timestamps) ZM1 to ZM4 and ZMG may be used, which may be stored inregisters and from which it is possible to determine time correctionquantities, for example the offset relative to the global time, or alsoa correction value for error handling, especially the correction valueaccording to the present invention. A time mark is a relative point intime which establishes the relationship between the relative time and anaction in the original bus (CAN controller). A time mark is representedas a register, a controller being capable of managing a plurality oftime marks. A plurality of time marks may be assigned to a message.

Reference message RN is the basis for the time-triggered, periodicoperation of TTCAN and arrow 112 indicates that reference message RN(111) is sent to the other stations (102 to 105). RN is clearlyidentified by a specific identifier and is received by all stations (inthis case 102 to 105) as clocking information.

FIG. 2 shows a system composed of a plurality of, and in this case two,TTCAN bus systems. 203B1 denotes therein a first bus, and 203B2 a secondbus. Two stations 201 and 202 are coupled to the first bus 203B1. Onestation 205 is coupled to the second bus 203B2. Station 200 is connectedto both buses 203B1 and 203B2 and acts as a connecting station or as agateway computer, or as gateway station or gateway controller, which hasaccess to both bus systems. Connection of the individual stations to therespective bus system is via an appropriate interface element, forexample interface element 210B1 in the case of station 201. Station 200is similarly connected as a gateway station via an interface element204B1 to bus 203B1 and via an interface element 204B2 to bus 203B2.Alternatively, in contrast to providing two interface elements 204B1 and204B2, it would be possible to provide one interface element that hastwo terminals for connection to bus 203B1 and to bus 203B2. Also shownin station 200 and 201 are a clock generator 211 and 206, respectively,and a timer component having an internal clock source or also a localtime basis 207 and 212, respectively; especially a crystal or anoscillator, especially a VCO (voltage controlled oscillator). Presentwithin the respective timer component 206 or 211 is a time-recordingcomponent, e.g., a counter 208 or 213, respectively.

Control functions in the respective station, especially for theinput/output of data on the bus system, for taking time information fromthe timer component, or also the calculation of the offset and thedetermination of the correction values for comparison of the time marksand for synchronizing the buses and bus stations, and further processesand process steps may be performed by components 209 and 214 formingprocessing components, especially a microcomputer or microprocessor oralso a controller. Parts of those functionalities or the entirefunctionality may, however, be present directly, that is, implemented,in the respective interface component. In addition to every otherstation, in this case the gateway station also may be specified as thetimer of the global time, that is to say, as the sender of the referencemessage, especially for both bus systems. Essentially, therefore, thesystem composed of at least two bus systems shown in FIG. 2 is to beregarded for each bus system in the same manner as the system having onebus system which is shown in FIG. 1, it being necessary for the variousbus systems to be synchronized, especially in applications where safetyis critical. As a result, the errors described in the introductionpropagate from one bus system to the other.

FIG. 3 shows the principle of exchanging data by time-triggered,periodic transmission of messages over time. That message transmissionis clocked by the timer of the respective bus system using referencemessage RN. By definition, basic cycle BZ may, as shown here, includethe actual time windows for messages ZF1 to ZF4 and also ZFRN, the timewindow for the reference message, and a pause period PZ. Cycle time ZZgenerally corresponds to the time for such a basic cycle.

For the sake of clarity, the time windows for data transmission ZF1 toZF4 will be referred to hereinafter as data cycle DZ, cycle time ZZincluding reference message RN in basic cycle BZ and also pause periodPZ, in this case from t0 to t6, t6 to t12, t12 to t18, and t18 to t24.This accordingly gives in the example illustrated 4 data cycles DZ0 toDZ3 similarly in 4 basic cycles BZ0 to BZ3.

As shown, from t0 to t1, t6 to t7, t12 to t13 and t18 to t19, i.e., intime window ZFRN, reference messages RN of the respective basic cyclesBZ0 to BZ3 are transmitted. The structure of the time windows ZF1 to ZF4that follow a reference message RN, i.e., their length in segments S,their number and their position in time, may be specified. It is therebypossible to form from a plurality of basic cycles with their associatedreference messages, data cycles and pause periods a total cycle GZ whichbegins at t0 and ends at t24, in order to be repeated again. Timewindows ZF1 to ZF4 include, for example, from two to five segments Seach having, for example, 16, 32, 64, etc. bit times. The messages sentare shown circled in FIG. 3 as RN and A to F. All of those messages ofall the stations of at least one bus system are organized as componentsof a matrix that represents the total cycle GZ, the so-calledcommunication matrix. That communication matrix is composed, therefore,of the individual cycles BZ0 to BZ3 with their associated referencemessage RN, corresponding data cycle DZ0 to DZ3 and associated pauseperiod PZ. Those data cycles DZ0 to DZ3 may optionally be made up ofexclusive and arbitrating components. For example, on the one hand, anentire time window, such as ZF3 here, may be specified as beingarbitrating in accordance with CAN or, alternatively, only a singleelement of a basic cycle or data cycle, such as, for example, from t15to t16. It is possible here for the arrangement of the messages (A to F)within basic cycles BZ and data cycles DZ to be freely specified. A timewindow ZFx is linked for exclusive components to a CAN message object,it also being possible for a time window to be left empty or used forarbitrating components.

According to an example embodiment of the present invention, therefore,a communication matrix in TTCAN is composed either of basic cycles withnothing but data cycles of equal length with reference message or it mayadd a pause, that is, a pause period PZ, in the data traffic after oneor more data cycles, which pause period (PZ) is ended, for example, byan event, such as a new reference message. In FIG. 3, in one particularapplication a pause period PZ is provided after every data cycle, i.e.,in every basic cycle, in this case therefore from t5 to t6, t11 to t12,t17 to t18 and t23 to t24. According to the present invention, it isalso possible, however, for such a pause period to be provided onlyafter one data cycle or after every even number of data cycles, that isto say, 2^(n) where n is a natural number. Similarly, such a pauseperiod PZ may also occur after every odd number of data cycles, that isto say, at 2^(n)+1, again with n as a natural number. Accordingly, thelength of a basic cycle or the duration of the cycle time ZZ is made upof the time window for the reference message ZFRN, the data cycle DZand, as shown here in FIG. 3, the pause period PZ, that is to say, of alarger fixed part and a smaller pause. This method allows adaptation ofthe cycle time by adaptation of pause period PZ. It is thereby possibleto compensate for faults, especially delays caused by messagerepetitions, in particular of reference message RN, during one or morecycles. A pause that is required after a data cycle, for example a pausenecessitated by the system, may additionally be obtained by lengtheningthat cyclic pause period PZ. For reasons of notation, pause period PZ isincluded in the basic cycle. Basic cycle BZ, however, may refer to ZFRNplus DZ and the pause period may be regarded as being added, which,however, merely corresponds to a different notation and is just asadvantageous to the concept of the present invention and cannot resultin any limitation.

In FIGS. 4, 5 and 6, the example embodiment of the present inventionwill be described once more in detail.

FIG. 4 a shows a normal procedure with reference message RN in basiccycle BZ without the use of the pause period PZ. At time ts, referencemessage RN(n) begins. At time tE1, RN(n) has been completely transmittedand is valid. At the end of cycle time ZZ, the next reference messageRN(n+1) begins at time ts1, which is then valid at time tEN, and thenext data cycle DZ follows. If, as shown in FIG. 4 b, a fault occurs, are-start of that reference message RN(n) takes place at time tNS. Thatis to say, a defective message identified by an error frame,specifically a defective reference message RN in the case of TTCAN, isrepeated at that time tNS, since it is not possible to do without thatreference message. That repeated reference message RN(n) is then validat time tE2, whereupon data cycle DZ then follows in basic cycle BZ.That repetition of the message results, however, in the affected cycletime ZZ being extended and in the corresponding basic cycle BZ beingshifted, in particular delayed. That is to say, at the actual start timeof the next reference message RN(n+1), ts1, the basic cycle BZ has notyet ended and does not end until time ts2. The start of the subsequentreference message RN(n+1) does not occur until TS2, thereby producing adeviation, especially a delay, of ts2-ts1 caused by the fault, that isto say, the associated cycle time and the end of the basic cycleaffected is extended by that time. Each of those errors or faults leadsto a further delay in the timing, with the result that those delays addup to an increasingly greater deviation from the nominal time. In thecase of two or more TTCAN buses that are to be synchronized, as shown inFIG. 2, such a delay on one of the bus systems must also be put intoeffect on the other bus system in order to achieve synchronism again,i.e., the time deviations, in particular delays, are added to oneanother on all the bus systems, thereby propagating the error or fault.With a cycle time ZZ of, for example, 900 bit times (ts1) and atransmission period for reference message RN of 55 bit times (tE1), ifthe re-start is able to take place after 40 bit times (tNS) there isaccordingly such a shift or delay due to the fault of precisely 40 bittimes (ts2-ts1).

In FIG. 5, which consists of FIGS. 5 a and 5 b, instead of the cycletime being extended and the constant basic cycle being shifted ifnecessary, cycle time ZZ will be composed of a constant time component,the very data cycle DZ and reference message RN, and of a variablecomponent, a regular pause or pause period PZ. Cycle time ZZ is dividedinto the fixed time in the error-free case from ts to tp of, forexample, 850 bit times and the pause time from tp to ts3 of, forexample, 50 bit times, once again producing in the example, as mentionedin the case of FIG. 4, the 900 bit times. Accordingly, as shown in FIG.5 a, from ts to tE1 once again the transmission of reference message RNtakes place, and from tE1 to tp the transmission of data cycle DZ whichis followed by pause period PZ from tp to ts3. The total basic cycle BZtherefore includes according to the present invention reference messageRN, data cycle DZ and pause period PZ, the cycle time then including apossible fault and a corresponding delay and hence being capable ofbeing kept constant. This means that the basic cycle according to thepresent invention of the TTCAN lasts from the beginning of the referencemessage to the end of a possible pause period PZ. It is not, however,necessary for a pause period to be appended to or included in everybasic cycle; a start of the new reference message RN(n+1) either occursafter the end of pause period PZ at time ts3, which has then beentransmitted and is valid at time tE3, or alternatively already occurs atthe end of data cycle DZ if no pause period is provided in a basiccycle, so that compensation of a deviation occurs in the next basiccycle having a pause period.

If, as previously in FIG. 4 b, a fault then occurs in FIG. 5 b and are-start of reference message RN(n) occurs at time tNS, which referencemessage has been transmitted and is valid at time tE2, and if basiccycle BZ accordingly does not begin until tE2 and that cycle DZ thenends at time tEB, it is possible to correct this by shortening pauseperiod PZ by tEB−tp=tNS−ts. The start of the subsequent referencemessage RN(n+1) then occurs, as intended, at time ts3 with a shortenedpause period. Thus, overall, the time provided for basic cycle BZ andthe pause period is shortened. It is possible to obtain a shortenedbasic cycle time, as is were, by reducing the duration of the pauseperiod, precisely to at least 0.

If the length or duration of pause period PZ is not sufficient tocompensate for the delay caused by repetition of the message, it is alsopossible for the time compensation to be distributed over several basiccycles and pause periods. That distribution may, on the one hand, bespecified according to any desired specifiable scheme: ⅔ to the firstpause period, ⅓ to the second pause period, or ¼ to the first pauseperiod, half to the second pause period, ¼ to the third pause period,and so on, or distribution may preferably be performed by equaldistribution, that is to say equal components corresponding to thenumber of pause periods within total cycle GZ. That distribution alsomakes it possible to keep that pause period small, since that time isnot available for communication over the bus and thus reduces thepossible bus utilization ratio. As shown in FIG. 3, the possibleshortening of the cycle time is taken into consideration in thecommunication matrix.

If such a fault occurs in the case of a plurality of interconnected andsynchronized bus systems, it is possible for the correction regardingthe time cycle to be carried out also on the other bus system, that isto say, on the bus system where the fault has not occurred, whichpermits great flexibility in error correction. Shortening of the pauseperiod may also take place on the bus system on which the fault has notoccurred, should that be necessary, for example for safety reasons. Itwould also be possible for the adaptation of the pause period forcorrection purposes to be distributed over a plurality of bus systemsand hence over a plurality of basic cycles of different bus systems.Using a plurality of bus systems therefore provides a very wide varietyof options for achieving synchronization by lengthening and shorteningpause period PZ. A pause period also does not have to be provided inevery basic cycle but may be provided, for example, at the end of every2^(n)th basic cycle or at the end of every 2^(n)+1 th basic cycle, wheren is a natural number (nεN), thereby allowing time compensation to bemade only at every second (odd or even) cycle in the event of a fault.

Establishing of the delay caused by message repetition of the referencemessage by determining a correction value is illustrated in FIG. 6. FIG.6 a shows the course of three successive basic cycles, showing in thiscase fault-free operation. Shown here are the start times of therespective reference messages RN1 to RN4, the times when the respectivereference messages RN1 to RN3 are valid and the beginning of therespective pause period PZ1 to PZ3. FIG. 6 a shows fault-free operationfor the exchange of data. FIG. 6 b substantially corresponds to FIG. 6 aexcept that here a fault occurs and the correction of that error bydetermination of a correction value is shown. The cycle time is updatedat each point in time when the reference message is valid, RNvalid. Thetime between the start of the defective reference message, in this caseR3, and the start of the successfully concluded reference message, thatis, RN3 Re-start, t76-t66, is ignored and the cycle time counter issynchronized with the beginning of the successfully concluded referencemessage. Accordingly, the delay caused by the fault cannot be seen atthe cycle counter. It is possible for that circumstance to be utilizedaccording to the present invention if the local time is considered inparallel. At any point in time in the basic cycle after the referencemessage has been received, both cycle time ZZ and local time LZ areread. To calculate the run-time since the last measurement, thedifference is found between the local time read and the measurement inthe previous basic cycle. As a comparison value, the difference betweenthe basic cycle length and the cycle time read out in the previous basiccycle is calculated and is added to the currently read cycle time. Thedifference between the calculated local time difference and thecalculated comparison value of the cycle time gives the delay due tomessage repetitions of the reference messages. The respective times inthe cycle time are denoted by C and the respective times in the localtime by L. Thus, the difference in the local time for determining thecorrection value in relation to the fault at t76 is given by:tLn=Ln+1−Ln or, in this case, tL2=L3−L2.

The comparison value for the cycle time is given by:tnComparison=(length of cycle time ZZ−Cn+Cn+1) or, in this case, byt2Comparison=900 bit times−C2+C3.

Thus, a correction value K of tL2-t3 Comparison is found. Thatcorrection value is the quantity for one-off or distributed shorteningof the regular pause period PZ.

1-16. (canceled)
 17. A method for exchanging messages containing databetween at least two stations over a bus system, comprising: repeatedlytransmitting over the bus system, by a first station, a referencemessage containing time information of the first station at at least onespecifiable time interval, the time interval being subdivided as a basiccycle into time windows, a pause period of variable duration beingprovided at an end of at least one basic cycle; transmitting messagescontaining data in at least some of the time windows; and adapting theduration of the pause period to change a time of a start of a next basiccycle.
 18. The method as recited in claim 17, wherein the time of thestart of the basic cycle is corrected by shortening the duration of atleast one pause period.
 19. The method as recited in claim 17, whereinat least two bus systems are synchronized with one another, a time of astart of a basic cycle of a first bus system is corrected by adaptationof the duration of the pause period of a second bus system.
 20. Themethod as recited in claim 17, wherein a pause period is provided at anend of every basic cycle.
 21. The method as recited in claim 17, whereina pause period is provided at an end of every 2nth basic cycle, where ncorresponds to a natural number.
 22. The method as recited in claim 17,wherein a pause period is provided at an end of every 2n+1th basiccycle, where n corresponds to a natural number.
 23. The method asrecited in claim 17, wherein, when data is exchanged, a pause period ofvariable duration is provided at an end of each of at least two basiccycles, by which a change of a start of a beginning of at least onebasic cycle is corrected by adaptation of the duration of the at leasttwo pause periods.
 24. The method as recited in claim 17, furthercomprising: determining a correction value based on a local time of astation and a cycle time, the correction value being used in adaptingthe duration of the pause period
 25. The method as recited in claim 24,wherein the correction value is determined from a first differencebetween two local times of the station in two successive basic cycles.26. The method as recited in claim 25, wherein the correction value isdetermined from a second difference between two cycle times of twosuccessive basic cycles.
 27. The method as recited in claim 26, whereinthe correction value is determined from a comparison value formed by asum of the time interval of the basic cycle and the second difference.28. The method as recited in claim 27, wherein the correction valuecorresponds to the difference between the first difference and thecomparison value.
 29. The method as recited in claim 24, wherein atleast two pause periods are provided in at least two basic cycles forexchanging data, and the correction value is distributed over the atleast two pause periods in a specifiable manner.
 30. The method asrecited in claim 29, wherein the correction value is evenly distributedover the at least two pause periods.
 31. A device for exchanging data inmessages between at least two stations connected by a bus system,comprising: a first arrangement at a first station configured torepeatedly transmit a reference message containing time information ofthe first station over the bus system at at least one specifiable timeinterval; a second arrangement configured to subdivide the time intervalas a basic cycle into time windows of specifiable length, the messagesbeing transmitted in the time windows; and a third arrangementconfigured to provide a pause period of variable duration at an end ofat least one basic cycle when data is exchanged, a start of a beginningof the basic cycle being corrected by adaptation of the duration of thepause period.
 32. A system having at least two stations for exchangingdata in messages between the at least two stations, comprising: a bussystem which connects the two stations; a first arrangement, at a firststation, configured to transmit the messages containing the data overthe bus system, the first station repeatedly transmitting a referencemessage containing time information of the first station over the bussystem at at least one specifiable time interval; a second arrangementconfigured to subdivide the time interval as a basic cycle into timewindows of specifiable length, the messages being transmitted in thetime windows; and a third arrangement configured to provide a pauseperiod of variable duration at an end of at least one basic cycle whendata is exchanged, a beginning of the basic cycle being corrected byadaptation of the duration of the pause period.